In manufacturing an LSI (large-scale integration), tungsten is widely used for a MOSFET gate electrode, a contact with source/drain, a word line of a memory, or the like. In a multilayer interconnection process, a copper wiring is mainly used. However, copper is easily diffused and has insufficient heat resistance. Therefore, tungsten is used for a portion where heat resistance is required or a portion where electrical characteristics may deteriorate due to diffusion of copper.
A physical vapor deposition (PVD) method has been conventionally used for a tungsten film forming process. However, it is difficult in the PVD method to deal with a portion where a high step coverage is required. Therefore, a chemical vapor deposition (CVD) method that provides a good step coverage is used for film formation.
As a film forming method for forming a tungsten film (CVD-tungsten film) by using the CVD method, there is generally used a method in which reaction WF6+3H2→W+6HF occurs on a semiconductor wafer as a target object by using tungsten hexafluoride (WF6) as a source gas and H2 gas as a reduction gas (see, e.g., Japanese Patent Application Publication Nos. 2003-193233 and 2004-273764).
However, in the case of forming a CVD-tungsten film by using WF6 gas, it is very concerned that a gate insulating film is reduced by fluorine contained in WF6 to deteriorate electrical characteristics particularly at gate electrodes or word lines of a memory in a semiconductor device.
As a source gas used for formation of a CVD-W film that does not contain fluorine, tungsten hexachloride (WCl6) is known (see, e.g., Japanese Patent Application Publication No. 2006-28572, and J. A. M. Ammerlaan et al., “Chemical vapor deposition of tungsten by H2 reduction of WCl6”, Applied Surface Science 53 (1991), pp. 24-29). Although chlorine also has reducibility like fluorine, the reducibility of chlorine is weaker than that of fluorine. Thus, it is expected that an adverse effect of chlorine on the electrical characteristics would be small.
Recently, as a semiconductor device is further miniaturized, it becomes difficult to fill a pattern having a complicated shape even by using the CVD method that can provide a good step coverage. In view of obtaining a higher step coverage, an atomic layer deposition (ALD) method, in which a source gas and a reduction gas are sequentially supplied with a purge process interposed therebetween, attracts attention.
However, in the case of forming a tungsten film by CVD or ALD, since the tungsten film has a poor adhesive strength to an oxide film such as an interlayer insulating film and requires a long incubation time, it is difficult to form the tungsten film. For the above reason, a Ti-based material film such as a TiN film or the like is used as an underlying film, so that a tungsten film has a good adhesive strength and a good fillability.
However, due to the recent miniaturization or complication of the devices, the adhesive strength and the fillability of the tungsten film may not be sufficiently ensured even by using a TiN film or the like as an underlying film.